Monkeys are classified as primates; they live in trees and on the ground and reside primarily in warm, tropical regions and rainforests. All monkeys fall into one of two main groups: Old World monkeys and New World monkeys. New World monkeys live in South America, while Old World monkeys live in Africa and Asia.
There are over 260 species of monkeys in the world, and there are many physical differences among them. Monkeys are similar to, but biologically distinct from, other primates such as apes and baboons. Most monkeys have tails, which help them swing through overhead tree canopies and move quickly among tree branches. Like humans, monkeys have opposable thumbs, which means that their thumbs have separate joints from the four other fingers of the hand.
Monkeys are social creatures and congregate in groups, which are called tribes, troops or missions. Most monkeys choose mates for life; after pairing, couples stay together to mate for successive seasons and share the duties of raising young. The smallest monkey is the pygmy marmoset, which grows to be the size of a squirrel upon reaching adulthood. The largest is the mandrill monkey, which can reach the size of an adult baboon when fully grown. Monkeys, like other primates, have fur-covered coats. They are highly intelligent and can learn new skills and use tools.